This invention relates generally to a method and a system for controlling forward transmit power in a spread-spectrum communications system, such as a code-division multiple-access (CDMA) system.
In a wireless communication systems, a mobile station may receive multiple forward channels, including a voice channel, a control channel, and a data channel. Forward channels are down link channels that are transmitted from a base station to the mobile station. The voice and data channels carry subscriber traffic, while the control channel sends data used to control the operation of the mobile station.
During a soft hand-off of the mobile station from one coverage area (e.g., sector) to another, the mobile switching center may authorize base stations to simulcast forward voice, control, and data channels over co-frequency carriers. Simulcasting refers to base stations that simultaneously transmit substantially the same information over multiple forward signals. Multiple base stations, which serve the geographic location of the mobile station, may transmit identical information on the forward voice and data channels to the mobile station. At the mobile station, the rake receiver combines the simulcasted channels to realize diversity gain, where possible.
However, the service provider may not have sufficient base-station resources to provide duplicate voice channels, control channels, and data channels for each base station participating in the soft hand-off. Further, the service provider may not have sufficient interconnect capacity, such as E1 or T1 lines, between base-station sites to handle duplicate voice channels for the soft hand-off. Even if the wireless system infrastructure has adequate channel capacity to completely support soft hand-offs, reducing the number of simulcasted channels participating in the soft hand-off can reduce interference and improve traffic capacity.
Accordingly, to achieve traffic capacity and interference reduction goals in a spread-spectrum wireless system, the wireless service provider may configure a soft hand-off to use what is commonly referred to as a reduced active set. In a reduced active set, the control, voice and data channels are not simulcasted from each base station participating in the soft hand-off. Instead, the data channels may be scaled back to transmit from as few as one base station. In practice, the voice and control channel for a mobile station are often multiplexed onto one Walsh coded channel of CDMA carrier, so the advantage of reducing the participation of voice and control channels is not as great as reducing the number of independent data channels participating in the soft hand-off.
The reduced active set has other implications besides interference reduction or traffic capacity enhancement. Because the control channel and the data channel are transmitted on a different number of forward carrier legs, the control channel and the data channel may experience differential Rayleigh fading. Accordingly, forward power control data can not be applied uniformly across the communications channels without a degradation in fading tolerance. Compensating for Rayleigh fading by changing down link transmit power is further complicated by the need for. sufficiently real-time response from the mobile station to the base station. Thus, a need exists to facilitate sufficiently rapid, independent power control of the forward link transmit power of the simulcasted voice, control, and data channels of participating base stations in a soft hand-off to adequately compensate for Rayleigh fading.
In accordance with one aspect of the invention, a method and system for controlling down link transmit power during a soft hand-off in a spread-spectrum wireless system supports independent power control of multiple forward channels to reduce fading. For a soft hand-off, a wireless system or a part thereof determines if a forward control channel is transmitted from different set of sectors of base stations than a forward data channel. For example, the mobile switching center determines if active control sectors are different in number or identity from at least one active data sector participating in the soft hand-off. Each sector is preferably represented by a distingishable pilot code of a pilot channel. If the forward control channel is transmitted from different sectors than the forward data channel, multiple reverse power control channels are allocated. The base stations adjust transmit powers of the forward channels grouped into different physical channels by receiving power control data over the allocated reverse power control channels to compensate for fading of the forward channels during a soft hand-off. In one embodiment of the invention, a distinct power control channel is associated with each forward data channel that is transmitted from a different set of sectors than any other channels associated the base stations participating in the soft hand-off.